In nuclear reactors designed with passive operating systems, the laws of physics are employed to ensure that safe operation of the nuclear reactor is maintained during normal operation or even in an emergency condition without operator intervention or supervision, at least for some predefined period of time. A Multi-Application Small Light Water Reactor project conducted with the assistance of the Idaho National Engineering and Environmental Laboratory, NEXANT and the Nuclear Engineering Department of Oregon State University sought to develop a safe and economical natural light water reactor. FIG. 1 illustrates a nuclear reactor design 5 that resulted from this project.
The nuclear reactor design 5 includes a reactor core 6 surrounded by a reactor vessel 2. Water 10 in the reactor vessel 2 surrounds the reactor core 6. The reactor core 6 is further located in a shroud 22 which surround the reactor core 6 about its sides. When the water 10 is heated by the reactor core 6 as a result of fission events, the water 10 is directed from the shroud 22 and out of a riser 24. This results in further water 10 being drawn into and heated by the reactor core 6 which draws yet more water 10 into the shroud 22. The water 10 that emerges from the riser 24 is cooled down and directed towards the annulus 23 and then returns to the bottom of the reactor vessel 2 through natural circulation. Pressurized steam 11 is produced in the reactor vessel 2 as the water 10 is heated.
A heat exchanger 35 circulates feedwater and steam in a secondary cooling system 30 in order to generate electricity with a turbine 32 and generator 34. The feedwater passes through the heat exchanger 35 and becomes super heated steam. The secondary cooling system 30 includes a condenser 36 and feedwater pump 38. The steam and feedwater in the secondary cooling system 30 are isolated from the water 10 in the reactor vessel 2, such that they are not allowed to mix or come into direct contact with each other.
The reactor vessel 2 is surrounded by a containment vessel 4. The containment vessel 4 is placed in a pool of water 16. The pool of water 16 and the containment vessel 4 are below ground 9 in a reactor bay 7. The containment vessel 4 is designed so that water or steam from the reactor vessel 2 is not allowed to escape into the pool of water 16 or the surrounding environment. In an emergency situation, steam 11 is vented from the reactor vessel 2 through a steam valve 8 into an upper half 14 of the containment vessel 4, and water 10 flashes as it is released through a submerged blowdown valve 18 which is located in a suppression pool 12. The suppression pool 12 includes sub-cooled water.
The nuclear physics and thermal hydraulics of a natural circulation nuclear power reactor are tightly coupled. The reactor core 6 generates the heat that creates the buoyancy needed to drive the flow through the loop. The flowing water in the reactor vessel 2 serves both as the reactor core coolant and as the fluid moderator that slows down the neutrons produced by the fission process in the reactor core 6. The fluid moderator temperature strongly affects the nuclear fission process that generates the heat in the reactor core 6. In turn, the fluid moderator temperature is governed by the reactor core power and fluid flow rate.
The tight coupling between the nuclear physics and the thermal hydraulics makes startup of a natural circulation nuclear reactor potentially unstable when the control rods are withdrawn to achieve core criticality to the point of adding heat to the fluid.
The present invention addresses these and other problems.